Speculation around Mr Phillips and Ms Sperling’s relationship began more than a year ago when they were spotted together at social events.
Most recently Mr Phillips and Ms Sperling attended Royal Ascot in June as guests of the King and Queen.
The couple took part in the traditional carriage procession the Royal Family make onto the racecourse to signal the start of the day.
Last month they were also present at Prince William’s charity fundraising polo matches and in the Royal Box at Wimbledon.
Ms Sperling has a daughter and previously wrote in Christian magazine Woman Alive, external about life as a single parent. She is reported to be a distant relation of the Duke of Gloucester through her late father.
Mr Phillips has two daughters, Savannah and Isla Phillips, from his first marriage to Autumn Phillips. They had married at St George’s Chapel in Windsor Castle in 2008 and divorced in 2021.
He is 19th in the line of succession to the British throne.
But Princess Anne rejected royal titles for her son and his sister Zara, and they are not “working royals”.